The illumination of a large area by a central lighting system has long been desired because of its many advantages. For example, a centralized light source is easier to maintain and can be more cost effective than distributed light sources. Heat generated by a centralized light source can be vented from the lighted area, whereas heat from distributed light sources is not easily vented. A light distribution system radiates minimal or no heat at locations remote from the centralized light source. The centralized light source can be sunlight and/or artificial light.
Light pipes or light conduits made of a transparent material having substantially planar inner surfaces and outer surfaces that are "in octature" have been utilized to transport light, as illustrated in U.S. Pat. No. 4,260,220 to Whitehead. These devices are typically constructed of an optical lighting film made of flexible polymeric sheets of a transparent material having a structured surface on one side and a smooth surface opposite the structured surface. The structured surface of the devices preferably include a linear array of miniature substantially right angles isosceles prisms arranged side-by-side to form a plurality of peaks and grooves. Further, the perpendicular sides of the prisms make an angle of approximately 45 degrees with the smooth surface. This structure of the polymeric sheets, as well as the shape of the light conduit, enables light to be constrained to travel through the light conduit without escaping through its walls if the angle by which the light rays deviate from the longitudinal axis of the light conduit does not exceed a critical angle. Thus, light entering a light conduit at an angle less than the critical angle is totally internally reflected.
The critical angle is defined as the arc sine of the ratio of the index of refraction of the surrounding medium (typically air) to that of the wall material. For example, for a transparent material of polymethylmethacrylate having a refractive index of 1.493, all incident light rays less than the critical angle of about 27.3 degrees, as measured along the longitudinal axis of the light conduit, will be totally internally reflected. On the other hand, incident light that enters light conduit outside the critical angle will not be totally internally reflected.
Alternatively, a light conduit can be constructed using a multi-layer optical film, such as disclosed in U.S. Pat. No. 5,661,839 (Whitehead). Light conduits have been constructed with various cross-sections, such as square cross-sections, as illustrated in U.S. Pat. No. 4,260,220, and circular cross-sections, as illustrated in U.S. Pat. No. 4,805,984.
In many applications, it is desirable to make light escape from the light conduit in a controllable manner. Many means for facilitating emission of light from the light conduit have been used in the past, such as disclosed in U.S. Pat. No. 5,363,470 (Wortman). In another example, an extractor such as a diffuse scatterer made from a highly reflective white polymeric tape such as SCOTCHCAL ELECTROCUT brand film, manufactured by Minnesota Mining and Manufacturing Company, St. Paul, Minn., may be placed inside a light conduit to increase the rate of leakage, or emission, of the light from inside the light conduit. The diffuse scatterer increases the rate of leakage by "scattering" light that hits it into non-total internal reflecting angular regions of the light conduit, thereby increasing the amount of light in those angles which allow light to be emitted from the light conduit. Typically, a strip of the highly reflective white polymeric tape is placed over the length of the light conduit to cause the scattering.
Current light distribution systems utilize a plurality of segmented light conduits attached to a light source, such as the LPS 1010 Light Pipe System available from Minnesota Mining and Manufacturing Company of St. Paul, Minn. Other components, such as end caps or junctions may also be connected to the segmented light conduits. The segmented light conduits can be joined to each other or to other components by a variety of techniques. U.S. Pat. Nos. 5,475,785 and 5,483,119 (Johanson) disclose overlapping the ends of adjacent light conduits to form a connector. Adhesive tapes can also be used to augment the connection. U.S. Pat. No. 4,805,984 (Cobb, Jr.) discloses light conduits that are telescopically tapered so that one conduit may be inserted into another. Alternatively, a sleeve of the same material may be placed over the abutting ends of two light conduits.
Most of the light weight materials typically used to construct light conduits have a relatively low hoop strength. Hoop strength refers to the ability of an object to retain its shape in opposition to an external force. Consequently, relatively small forces placed on the light conduit, particularly near a joint with an adjacent light conduit, can cause buckding that may separate the joint, allowing contaminants to enter the light distribution system or otherwise disrupting the light path. Misalignment during installation and thermal expansion/contraction after installation can also create stress in the light distribution system that may cause joints to separate. Moreover, depending upon how the light conduits are mounted, differential sagging of adjacent conduits can cause the joint to separate. Misalignment of adjoining segments can lead to reduced light transmission and contaminants entering the light conduit. The high static charge on some light conduits attracts dust that can migrate through extremely small openings between adjacent light conduits.
Optimum operation of a light distribution system depends on a variety of factors, such as accurate alignment of adjacent light conduits and preventing environmental contaminants, such as dust, moisture or insects, from entering the light conduits. Some prior methods of joining light conduits failed to provide the necessary structural support to maintain accurate alignment, especially in sections in excess of 20 meters in length. Additionally, maintaining rotational alignment is articularly important for light conduits that are designed to leak light along selected surfaces. Therefore, what is needed is a coupling system for light conduits that maintains both structural integrity and excludes environmental contaminants.